My mother passed away when my sister and I were 15 and 13, respectively. Tough time for my Dad to become a single parent. Through conversations with him later in life I realized how much he struggled with the "girl" talks with us. He did enlist the help of our neighbor, she would often invite us over for tea and casually bring up the girl stuff my Dad thought we needed to know. But, he also tried very hard to overcome his feelings of discomfort and have open and honest discussions with us also.
So, from my experience as the daughter - please try very hard to get past the awkwardness. Your daughters know that these are tough topics for you, but pushing through and talking about them will make you closer. Ensure that they know that they can come to you with anything and that if you do not know the answer you will find it for them. Oh, and read - go to the local library and get books on all this stuff - they may be written from a woman's perspective - which would be a good thing - and help you to be able to communicate with your daughters.
As for your 16 year old daughter - it time to have the birth control and sexual consequences talk with her. Yeah, makes you cringe, I know. My Dad had that talk with my sister when she was 16, as a "bonus" he included me, because, as he put it, "I don't think I can do this twice".
Enlist the Aunts - an hour away is not so far. Talk to them about your concerns, get pointers from them, and ask them to forge a stronger relationship with your daughters so they too can be part of the dialogue.
Are their any neighbors that you are close to? Our friend's Moms were invaluable to us - they often lent an ear and a life lesson when we needed it. So get to know the Moms of your girls friends also.
Basically, expand your network of Moms - starting here is good :) so that you build a base of support for your girls and yourself.
Do you daughters help you on the farm? While I grew up in the city - my Dad often had us do things with him - non traditional not girly things - skeet shooting, fishing, projects around the house, stuff he needed to do, or liked to do. Anyhow, those times sometimes led to some of the best conversations that we had - it was easier to talk about sensitive topics when we were shooting or casting than sitting at the kitchen table. This is a technique I use with my teen son - most of our conversations on these topics are while we are doing something else. There is something very intimidating about sitting down at the table, facing each other, and talking about sex. LOL
Just keep trying. Let them know that you are there for them. Keep the lines of communication open.
Good Luck
God Bless